1. Field of the Invention
The invention relates to contactless power or energy transmission systems and, more particularly, to contactless power transfer systems for transmitting electrical power from one table, desk or the like to another in the absence of requiring direct physical contact or any type of physical electrical wires, bus bars or the like.
2. Background Art
Contactless energy transmission systems are known in the prior art for transferring electrical power or energy from one device to another, without requiring any type of mechanical or physical, electrical connections. Contactless power transfer systems can exhibit a number of advantages over what may be characterized as conventional systems using electrical wires, bus bars or the like for power transfer. For example, contactless power transfer systems (excluding extremely high voltage configurations such as those found with transformers associated with utility power companies) are generally safer. This is because there is relatively limited danger of sparks or electrical shocks in view of the isolation of the initial power supply. Also, components of contactless power transfer systems tend to exhibit longer life, because there are few electrical contacts or other physical interconnections which can become worn and lose conductivity paths.
One type of contactless power transfer system uses what could be characterized as magnetic induction for purposes of transferring energy. Magnetic induction (also referred to as electromagnetic induction) is the production of voltage across a conductor which is situated in a changing magnetic field, or a conductor moving through a stationary magnetic field. The electromotive force (EMF) produced around the closed path is proportional to the rate of change of the magnetic flux through any surface bounded by that path. In practice, this means that an electrical current will be induced in any closed circuit when the magnetic flux through a surface bounded by the conductor changes. This applies whether the field itself changes in strength, or the conductor has moved through the field. Electromagnetic induction underlies the operation of all generators, electric motors, transformers, induction motors, synchronous motors, solenoids and many other electrical devices.
In particular, magnetic induction or electromagnetic induction is used with power transfer systems employing transformers. A transformer can be characterized as a device which transfers electrical energy from one circuit to another through inductively coupled conductors (i.e. the transformer coils). A varying current in the first “primary” winding creates a varying magnetic flux in the transformer's core. This produces a varying magnetic field through the secondary winding. This varying magnetic field induces a varying electromagnetic force or voltage in the secondary winding. This effect is often referred to as mutual induction.
If a load is connected to the secondary winding, an electric current will flow in the secondary winding, and electrical energy will be transferred from the primary circuit through the transformer, to the load. In an ideal transformer, the induced voltage in the secondary winding is in proportion to the primary voltage, and is given by the ratio of the number of turns in the secondary winding to the number of turns in the primary winding. Accordingly, by appropriate selection of the turn ratios, transformers allow an alternating current voltage to be “stepped up” by making the number of turns in the secondary winding greater than the number of turns in the primary winding. Alternatively, the voltage can be stepped down by making the number of turns in the secondary winding less than the number of turns in the primary winding. In the vast majority of transformers, coils are wound around a ferromagnetic core, with air-core transformers being an exception.
Transformers can come in a range of sizes, from a thumb nail-sized coupling transformer hidden inside a stage microphone, to relatively large units weighing hundreds of tons used to interconnect portions of national power grids. Although differing in sizes, all of these transformers operate with the same basic principles, although size ranges are wide and varied. Transformers are still found in nearly all electronic devices designed for household voltage. Also, transformers are essential for high voltage power transmission, which make long distance transmission economically practical.
Ideal transformers would have no energy losses, and would therefore be 100% efficient. However, in practice, transformer energy is dissipated in windings, core and surrounding structures. These losses can exist with respect to winding resistance. That is, current flowing through the windings causes resistive heating of the conductors. At higher frequencies, skin effect and proximity effect create additional winding resistance and losses. Also, hysteresis losses can occur. Specifically, each time the magnetic field is reversed, a small amount of energy is lost due to hysteresis within the core. For any given core material, the loss is proportional to the frequency, and is a function of the peak flux density to which it is subjected.
Still further, eddy currents can cause power losses. Magnetic materials are good conductors, and a solid core made from such a material constitutes a single short circuited turn throughout its entire length. Eddy currents therefore circulate within the core in a plane normal to the flux, and are responsible for resistive heating of the core material. The eddy current loss is a complex function of the square of supply frequency and inverse square of the material thickness. Other losses which may exist with respect to transformers are typically referred to as magnetostriction, mechanical losses and stray losses. Stray losses can exist with respect to any leakage flux which intercepts nearby conductive materials, such as the transformer's support structure. Such flux interception will give rise to eddy currents, and will be converted to heat.
With contactless power transfer systems relevant to this application, power from a primary winding in the power supply is transferred inductively to a secondary winding located in another physical location. Because the secondary winding is physically spaced from the primary winding, the inductive coupling occurs through the air.
With respect to the use of power in commercial and industrial establishments, power is typically generated from an outside power line from a utility company. Commercial and industrial establishments may often have meeting rooms or the like where a number of different tables, desks or other worksurfaces may be utilized. It is advantageous in many settings for the tables and the like to provide users sitting or otherwise working at the tables to have close access to electrical power. For these reasons, it is known to have various types of power centers mounted on or within the various worksurfaces. Such power centers usable with worksurfaces are disclosed in: Timmerman, U.S. Pat. No. 5,575,668; Byrne, U.S. Pat. No. 6,028,267; and Byrne, U.S. Pat. No. 6,290,518. The power centers disclosed in these patents obtain electrical power from a physically separate source through the use of electrical cords and the like. That is, electrical power is provided to the power centers through the use of physical and electrical connections.
A disadvantage of requiring physical, electrical connections to power sources and power centers having electrical outlets and the like on the tables is made apparent when the tables need to be rearranged and moved on a regular basis, so as to accommodate different sizes and types of meetings. Currently, these tables have to be mechanically and electrically connected by means of mechanical connectors, and male and female plugs and sockets. Methods of electrically connecting together power centers on various tables is not only relatively complex, but is also very time consuming. Accordingly, it would be advantageous to have an improved method of providing power to a series of tables, where the tables might be required to be arranged in various configurations.
Further with respect to contactless power transfer systems, a number of different types of systems exist. For example, Baarman, et al., United States Publication No. US 2008/0001572 discloses a vehicle power interface having an adaptive inductive power supply. The power supply includes a primary winding with a remote device holder. The inductive power supply is capable of providing power to remote devices placed within the remote device holder. Communications interfaces can be provided which enable communication between the remote device and any data bus within the vehicle.
A device for charging batteries is disclosed in Brockmann, U.S. Pat. No. 6,028,413. The device includes a mobile electrical device in a charging unit. The electrical device and charging unit inductively transfer electrical power by means of alternating magnetic fields from at least one primary winding to at least one secondary winding in the mobile device.
Mizutani, et al., U.S. Pat. No. 6,756,697 discloses a mounting structure for mounting accessories on an interior member in a vehicle compartment. The structure includes mounting portions which have non-contact type power sending terminals and vehicle-side antennas. The mounting portions transmit power from the battery of the vehicle by means of the non-contact type power sending terminals, and also transmit multiplex signals which include control signals required for controlling a number of accessories associated with the vehicle.
Baarman, U.S. Pat. No. 7,212,414 discloses a contactless power supply having a dynamically configurable tank circuit which is powered by an inverter. The power supply can be inductively coupled for one or more loads, and the inverter can be connected to a DC power source. When loads are added or removed from the system, the contactless power supply is capable of modifying the resonant frequency of the tank circuit, the inverter frequency, the inverter duty cycle, or the rail voltage of the DC power source.